This invention relates to a hinge joint assembly for flexibly connecting rigid structural components in the construction of modular buildings, such as those shown and described in my prior U.S. Pat. Nos. 4,074,477 and 4,145,850. Those building structures are formed from repeating alternating mirror-image forms of basic mathematically determinate structural modules. The structural modules are foldable. The joint assembly of the present invention permits the fastening of structural components such as struts (rods, tubes, or the like) about a central hinge body or hub.